This invention relates to the production of liquid aerosol particles having narrow size distribution. Particularly, this invention relates to methods and apparatus for the production of aerosol particles resulting from a liquid droplet feed source. In another aspect, this invention relates to methods and apparatus for dispersing a multiplicity of droplet columns by impinging these droplets onto a vibrating surface to create a high density fountain of aerosol particles.
Liquid aerosol particle materials have diverse applications such as creating mists and fogs, and specific particulate compositions and sizes, painting and coating, humidifying air, administering medicinal drug compounds by nasal inhalation, facilitating chemical reactions, and creating monodispersed ceramic particles from precursor organic metallic liquid. Aerosol standards, consisting of monodispersed aerosols of a high degree of monodispersity and accurately known particle size are needed for a variety of purposes ranging from fundamental aerosol research to chemical reactive applications as well as coatings and the like. To be generally useful, the aerosol production must be sufficiently flexible to permit aerosols to be generated from a variety of solid and liquid materials having desired physical properties.
Chemical reactions involving small liquid aerosol droplets provide a basis for producing particulate products in the form of small size solid particles. Such small aerosols also offer a large surface area to the carrier gas in which they are suspended or transported, hence are advantageous in promoting liquid gas chemical reactions or liquid droplet evaporation.
Conventional monodispersed aerosol generators such as the spinning-disk aerosol generator, the atomizer-impactor generator, the condensation generator, vibrating orifice aerosol generators all have severe limitations for use as a basic aerosol control particle size generator. The spinning-disk generator, while flexible in terms of aerosol material, can produce aerosols with geometrical standard deviations of not better than 1.07 or a standard deviation of approximately 7% of the mean particle size. The spinning-disk generator is also a relatively complex piece of apparatus.
Another conventional method for producing small liquid aerosol particles is by means of ultrasonic excitation in a manner to produce mechanical disruption of a bulk source liquid through the physical process of cavitation. Cavitation in liquids can cause aerosol droplets to be ejected from the liquid surface to become suspended in a surrounding carrier gas or atmosphere. This technique has been reported and used previously to generate liquid droplets in a size and range of 1 to 3 micrometers for applications where high-density concentrations of liquid aerosols are required. A particular reference to this technique is provided in a paper entitled "An Improved Ultrasonic Nebulizer System For The Generation of High Density Aerosol Dispersion" by M. B. Denton and D. B. Schwartz published in Review of Science Inst., Vol. 45, Pages 81 through 87, January, 1974. For the particle size range size set out above, the ultrasonic cavitation frequency must be approximately 3 MHz and, using a piezoceramic drive element, the excitation power required to produce cavitation is approximately 100 watts. When a chemical substance or solution is diluted in a solvent and subject to cavitation, aerosol products are produced. The solvent will evaporate from each particle to produce a residual particle whose diameter is reduced from the original aerosol particle diameter by the cube root of the volume concentration of the chemical dilutant. Thus, for example, by using a source liquid solution whose solid precipitate components are diluted in the range of 100 to 1 to 1,000 to 1 by the volatile solvent, the resulting solid particles after solvent evaporation will be in the range from 1/5 to 1/10 the diameter of the original liquid aerosol droplet.
A method of preparing a fine-particle oxide salt from oxculates, acitates, and carbonates is to thermally decompose, pyrolyze, or hydrolyze them to their respective oxides. Current results obtained using the controlled hydrolysis method are the most successful to date in producing accurately sized particles having spheroidal shape and a diameter of approximately 200 nanometers agglomeration of particle.
Size dispersion of particles no better than plus or minus 10% can be produced by either: (1) precipitation of supersaturated solutions of an appropriate water reactive metal alkoxide or (2) preparation of a monodispersed aerosol by condensation of a super saturated atmosphere of organometallic vapor in a flowing gas stream. Upon dispersal in water to form a colloidal suspension, only disordered colloids, which have a tendency to aggregate upon removal of the suspending solvent phase during slip casting, will be present. These powders still exhibit significant agglomeration to induce occasional preferential sintering defects in the finished microstructure of the ceramic, resulting in local variations in density and potential stress concentration centers.
Furthermore, many of the newest and most promising ceramics such as silicone nitride are not processable by controlled hydrolysis of alkoxide solutions. For these ceramics, other methods of particle formation are required.
One liquid aerosol production method and apparatus particularly adapted for producing mono-size ceramic particles is found in the teachings of U.S. Pat. No. 4,801,411, Wellinghoff, et al., hereby incorporated by reference. The '411 patent discloses the use of ultrasonic cavitation to produce aerosol from a bulk source liquid. The basic process of aerosol production by bulk liquid cavitation is also described in the Denton & Schwartz article, "An Improved Ultrasonic Nebulizer System For The Generation Of High Density Aerosol Dispersions". These methods are effective largely because the organometallic compounds are liquid solutions in which controlled hydrolysis produces uniform microsized particle precipitation. Alternative mechanization of the alkoxide hydrolysis reaction and precipitation process, using aerosol particle formation and flow stream processing, provide a high degree of particle handling, thereby avoiding agglomeration. Production of the organometallic precursor liquids as uniform size aerosol droplets and subsequent electrostatic charging and electromagnetic separation of these particles assures high uniform size and lack of agglomeration during subsequent reaction to form, for example, finished molding powder.
The '411 patent discloses the introduction of a bulk liquid on to a piezoelectric transducer. The mechanical resonance of the transducer causes cavitation of the bulk liquid and the release of aerosol at the liquid surface. In another embodiment, the patent provides a methodology wherein the bulk liquid is allowed to separate and form into two components before the transducer is operated. The resulting aerosol particles are comprised of one component coated by the second component liquid. Thus, the '411 patent is particularly suited to the production of two phase aerosol particles produced by cavitation of a separated and stratified bulk liquid.
Unfortunately, the teachings of the '411 patent and other publications do not completely transfer all of the bulk liquid into aerosol form. Small quantities of aerosol therefore cannot be produced without significant waste. Splashing and agglomeration of the bulk liquid make it difficult to produce uniformly small aerosol particles. The efficiency of the known methods and apparatus depend upon the mass of the bulk liquid and changes in the mass as aerosol is produced, because of mass loading effects on the resonance frequency of the vibrating transducer.
Presently, source liquid is ultrasonically excited to cavitate in bulk form, causing a violent mechanical disruption of the liquid including vigorous splashing of large non-aerosol droplets. This violent disruption produces a wider range of particle sizes and may be desirable or acceptable in certain applications. However, these wide range particle size interfere with the production of uniform smaller control particle sizes. Improved methods are needed to efficiently produce small liquid aerosol particles having a narrow size dispersion range.